From our main office we use Tax invoice for B2B invoices. & Retail Invoices for B2C we are adding another offline location which will be billing manually then added here

I'd be glad to assist you today, @info1178.

To clarify, are you trying to customize or add a location field of your invoices? If so, you can edit your invoice template to do it.

Please check out this article for details about customising invoice templates: Customise invoices, estimates, and cash memos in QuickBooks Online.

We also have a guide on how to customise invoice for GST which I'm sure you'll find helpful. It has details that'll help you create an invoice to make it GST complaint.

Feel welcome to comment below if you're referring to something else or if you have additional information. I'm always here to lend a hand. Thanks for posting and have a wonderful day ahead.

Hi 

I am trying to add a third series of invoices. like currently my invoice Tax Invoices run TI -1001 onwards & Retail invoice RI - 1001 onwards.

Now as some billing will happen manually at an offline location, there in the bill book I will be running a different Numbering series - FO - 1001.

Is there an option to create that as a standard drop down or I will have to manually change every time I make that invoice in Quickbooks.

Thanks for the additional detailed information, info1178.

You'll have to manually enter the invoice number everytime you enter a different one while the option to have selection of Invoice numbers is unavailable. I'll take note of this suggestion so we can improve our system more. 

To keep track of the next invoice you'll create for that department or section, you can use the Recent Invoices option at the upper-left of the Invoice paege. You can look at the screenshot below for your reference. 

Aside from that, you can check out our Class or Location Tracking features that will help you organise customer and supplier transactions. You can use this articles as reference: Get started with class tracking in QuickBooks Online.

I'm always here to help you manage your transactions. If there's anything else that you need help with, just mention me in your reply. You take care!

Dear Cathrine 

I want to use different invoice format for the Tax Invoice & Retail invoice. how to do the same. 

As Tax invoice address is different and the location for Retail invoice is separate.

You can create a new invoice format, info1178.

1. Click the Gear icon.
2. Select Custom Form Styles.
3. Select New Style.
4. Select Invoice.

Check this article to know more about customizing invoices: Customise invoices, estimates, and cash memos in QuickBooks Online.

Here's another article: Customise sales forms by location. So you can track sales by location.

Let me know if you need further assistance.